Characteristics of coupled-mode optical filters

Abstract

We provide a quantitative picture of the characteristics of a CdS coupled-mode band-pass optical filter. The operating principle of these coupled mode filters (CMF) is the energy exchange between the ordinary and extraordinary modes when the sample, inserted between two crossed polarizers ∥ and ⊥C, is strained by uniaxial stress at 45° from the C axis. The filter operates at the isotropic wavelength λi where ne=n0 ensures phase matching. The transmission rate and the selectivity are investigated as functions of the stress magnitude T and the sample thickness e. The best results are obtained by adjusting T so that the coherence length Lc=2e. For a 1-mm-thick crystal, the half-height passband is ∼4 Å, while the transmission rate is ∼70% at 77 °K and 20% at 300 °K. The large field of view, in practice ∼45°, constitutes a significant advantage over interference filters. We compare the CMF with zero crossing birefringent filters (ZCBF). Around λi, the characteristics of both filter types are rather similar, but the single transmission window of CMF gives the latter a distinct advantage over ZCBF. Transmission control by variation of applied stress is an added advantage of CMF.